Isotopic evidence of cool winter conditions in the mid-Piacenzian (Pliocene) of the southern North Sea Basin Annemarie Valentine a, ⁎, Andrew L.A. Johnson a , Melanie J. Leng b , Hilary J. Sloane b , Peter S. Balson c a Geographical, Earth and Environmental Sciences, School of Science, University of Derby, Kedleston Road, Derby, DE22 1GB, UK b NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK c British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK abstract article info Article history: Received 13 December 2010 Received in revised form 20 April 2011 Accepted 10 May 2011 Available online 18 May 2011 Keywords: Palaeoclimate Pliocene Mid-Piacenzian Sclerochronology Oxygen isotopes Bivalves Oxygen isotope thermometry of Aequipecten opercularis and Atrina fragilis bivalves (which demonstrate all- year growth), provides quantitative evidence of cool-temperate winter conditions (below 10 °C) during deposition of the mid-Piacenzian Oorderen Sands and time-equivalent strata in the southern North Sea Basin (SNSB). Isotopic summer temperatures (seafloor) are within or only marginally above, the cool-temperate range (upper limit 20 °C). The occurrence of warm-temperate dinoflagellate cysts alongside (and presumably contemporaneous with) the bivalves, indicates the development of a correspondingly warm surface layer in summer (encystment allowing the dinoflagellates to survive cool winter conditions).This evidence of greater surface seasonality than now is consistent with greater global warmth and a reduction in the vigour of the Gulf Stream/North Atlantic Drift (GS/NAD), leading to a reduced supply of winter heat. This reduction in GS/ NAD strength may be linked to breaching of the emerging Isthmus of Panama. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Numerical computer models predict that global mean surface temperatures (GMST) will be several degrees celsius higher than now by the end of the 21st century, due to the impact of increasing atmospheric CO 2 (Jansen et al., 2007). As the most recent interval with a similar GMST and atmospheric CO 2 level (Haywood et al., 2011), the Pliocene is a favourable test-bed for these models. It would be inaccurate to apply Pliocene climatic state as a direct analogue for future global climate, as Pliocene topography, ocean gateways and vegetation patterns were different from present conditions (Haywood et al., 2011; Robinson et al., 2011-this issue). Proxy investigations into Pliocene palaeoclimate provide the opportunity to evaluate model retrodictions on such specifics as regional climate, and the results of these tests measure the predictive capabilities of the models. The Pliocene Research, Interpretation and Synoptic Mapping (PRISM) group has been a major source of such proxy data (Dowsett, 2007; Dowsett et al., 1999, 2009). The PRISM group's information on sea-surface temperature (SST) has been largely derived from analysis of microfossil assemblages, with a focus on the mid-Piacenzian (c. 3.3–3.0 Ma) (Dowsett, 2007; Dowsett et al., 1999, 2009). The research has identified particularly warm conditions during this interval (GMST 2–3 °C higher than present), leading to the concept of a ‘Mid-Piacenzian Warm Period’ (MPWP) (Dowsett et al., 2009, 2010). Microfossil-assemblage data indicate that times of warm condi- tions were interspersed by cool episodes and these have been confirmed by foraminiferal oxygen-isotope data (Lisiecki and Raymo, 2005). Cooling may have been a global phenomenon but analysis of dinoflagellate-cyst assemblages from the eastern North Atlantic suggests that in this region it was due to reduction in heat supply through the North Atlantic Current/Drift (NAC/NAD) (De Schepper et al., 2009a). It is important to note that within the Pliocene tectonic changes occurred, leading to the development of the Isthmus of Panama (Keigwin, 1982; Lunt et al., 2008; Steph et al., 2010). It is thought that this would have increased ocean heat transport within the NAC (Lunt et al., 2008). By the MPWP, some evidence suggests that the Isthmus was fully closed (Keigwin, 1982). Nonetheless, the dates of a fully formed Isthmus of Panama remain disputed. Schmidt (2007) suggests that the barrier was likely intermittently breached in the Pliocene, potentially impacting on the volume of heat transferred by the NAC. Here, we investigate mid-Piacenzian marine temperatures at another location in the east Atlantic region, the eastern part of the southern North Sea Basin (SNSB). PRISM2 (Dowsett et al., 1999; Haywood et al., 2007) datasets for the mid-Piacenzian North Sea Basin indicate winter temperatures above 10 °C (i.e. a warm-temperate regime as defined by Krantz, 1991), as compared to the present cool- temperate climate (winter temperatures below 10 °C) (Krantz, 1991). Additional dinoflagellate microfossil evidence for the mid-Piacenzian Palaeogeography, Palaeoclimatology, Palaeoecology 309 (2011) 9–16 ⁎ Corresponding author. Tel.: + 44 1332 591191; fax: + 44 1332 593031. E-mail address: A.Valentine@derby.ac.uk (A. Valentine). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.05.015 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo